flask-openapi3 looks good but has only 246 stars. Would be worried using it in production. flask-pydantic has no openapi tie-in. Oh look, there's me bumping the openapi request that's been an issue since 2020:
https://github.com/pallets-eco/flask-pydantic/issues/17 which has an open PR since 2022.
It's possible between Quart and svcs (for DI) and some Pydantic/Marshmallow/OpenAPI extension you might be able to mimic what FastAPI does. But I'd just use FastAPI. I use async too. It's a lot easier to scale in my opinion.
Do none of these pieces matter to you? Like do you not do any data validation or care about OpenAPI?